1. Field of the Invention
The present invention relates to an optical line distribution frame which can be used in a telecommunication exchange, in particular when information bit rates are very high. The distribution frame of the invention is principally an all-optical distribution frame but some functions can be implemented by conversion to an electronic mode followed by return to an optical mode.
2. Description of the Prior Art
In the field of distribution of lines, also referred to as switching or cross-connection, the function of a distribution frame is to enable a signal conveyed by one of N input lines of the distribution frame to be routed to one of Nxe2x80x2 output lines of the distribution frame. To simplify the description it is assumed that Nxe2x80x2=N, firstly because a call normally requires the same number of calling and called parties and secondly because it can be shown that any other organization can be reduced to an N by N type distribution.
In an all-optical distribution frame the N input lines are optical lines, i.e. individual optical fibers. As an alternative to this, one fiber can convey a plurality of signals simultaneously using wavelength division multiplexing. The signals conveyed by the individual optical fibers can be amplitude-modulated onto carriers with the some wavelength for all of them or with different wavelengths.
A distribution frame core normally includes frequency domain cross-connection modules. To this end, all the separate optical signals terminating at a cross-connection module modulate carriers with different wavelengths. The signals are therefore xe2x80x9ccoloredxe2x80x9d by the different wavelengths. The input of a cross-connection module amalgamates or mixes all the signals to be distributed at the same time and distributes the whole of this combination each time between a plurality of output channels. Frequency domain filters in each channel select a single wavelength, i.e. a single optical signal. The combination of the coloring function, the cross-connection function and the filter function achieves the required selective routing.
However, frequency domain cross-connection means that the energy distributed between the channels is shared, and therefore reduced in each channel, simply by virtue of the fact that all the signals are present in all the output channels.
Frequency domain cross-connection is complemented by spatial switching using space switch modules to complement frequency domain cross-connection, in particular to prevent too great a loss of energy if the number of output channels is too high. A space switch can include a mirror which reflects an optical signal emanating from a termination of an optical fiber to one of K terminations of receiving optical fibers. A K by K switch would therefore include K mirrors. It is equally feasible to connect optical fiber ferrules directly to each other. A space switch module is normally opto-mechanical whereas a frequency domain module is all-optical or opto-electronic. All-optical solutions, i.e. solutions with no mechanical moving parts, can be envisaged for spatial cross-connection.
An architecture of the above kind gives rise to two problems. Firstly, frequency domain cross-connection leads to high losses and requires the optical signal to be regenerated before subsequent routing. Secondly, frequency domain cross-connection requires coloring devices whose function is to convert a signal conveyed by a wave at a wavelength xcexi into a signal conveyed by a wave at a wavelength xcexi. All-optical converters, or more generally opto-electronic converters, of this type are known in the art. These converters are the least reliable components in a distribution frame. They break down. To prevent the harmful consequences of these breakdowns, the circuits of a normal distribution frame include converters which are redundant compared to the number of optical signals to be processed.
For example, FIG. 1 shows a prior art distribution frame in which an input block receiving P optical signals includes P converters IWTxcex1nxc2x01 to IWTxcex1nxc2x08 (for simplicity P=8 in this example). To enable the addition of a redundant converter IWTxcex1nxc2x0p (xe2x80x9cpxe2x80x9d signifying xe2x80x9cprotectionxe2x80x9d), the converters must be preceded by a P to P+1 switch (here an 8 to 9 switch) and followed by a P+1 to P switch. In the solution shown, an input block therefore provides P (8) signals at a wavelength xcex1. Other input blocks among the M available blocks (M=16 in this example) produce signals with wavelengths from xcex2 to xcexM. Each of the P outputs of an input block is assigned one of P ranks i. The N=Pxc3x97M outputs of the M input blocks are connected to the inputs of P star couplers each of which has M inputs of a distribution core C. However, there can be a greater number of star couplers if each of them has fewer inputs. A coupler has the same number of outputs and inputs. The assigned outputs are of rank i.
For example, a first star coupler receives signals from all outputs of rank 1 of the output switches of the input blocks. A final star coupler, coupler number P (number 8), receives signals from the inputs of rank P of the output switches of the input blocks. In other words, each star coupler receives at its inputs signals with different wavelengths. A coupler of this kind therefore mixes all the signals and distributes them to all its outputs. The mixing involves no risk of degrading the quality of the signals since their colors (wavelengths) are different. Nevertheless, and due entirely to the fact that the signals are distributed between a large number of outputs, the energy that can be distributed is inevitably reduced in proportion to the number of outputs.
It follows from what has already been stated that cross-connection can advantageously be complemented by space switching. In this example, all the output channels of rank i of the P star couplers are connected to P inputs of a space switch of rank i. In practice a space switch of this kind therefore receives at its input P mixes of signals colored by wavelengths xcex1 to xcexM. In an architecture of the above kind a space switch therefore switches groups of signals, i.e. the mixes, rather than individual signals.
The outputs of the space switches are connected to filters for extracting a single wavelength in each mix. The filters and the space switches are controlled in accordance with orientation commands OR processed by a central control unit G.
Output blocks take the signals from the filter outputs and color them with a wavelength suited to their subsequent routing. Like the input blocks, the output blocks include converters. Theses converters suffer from the same lack of reliability as the input converters. They are also complemented by redundant converters.
From the practical point of view, for reasons of reliability, even the distribution core C is duplicated. Thus all the output switches of the input blocks, the star couplers, the space switches, the filters and the input switches of the output blocks are present twice over.
Various technologies are feasible for these various units. If the technology of the output switches of the input blocks and the input switches of the output blocks is a switching technology, energy losses are incurred of the order of 4 dB for each signal. If the technology is a broadcast technology (of the kind used in a star coupler) the losses are higher. The losses depend on the number of outputs and therefore on the number of inputs of the switch. The loss is 6 dB if this number is equal to eight, as shown here.
A star coupler has the same disadvantages and, especially if it is a 16 by 16 coupler, its transmission loss for each signal transmitted on each line is 12 dB. The space switch has a loss of 9 dB. Simplifying, it can therefore be assumed that a distribution core like that shown in FIG. 1 causes a loss of 29 dB on each signal. This loss can be compensated, in particular in the converters of the input blocks and the output blocks, by amplification performed simultaneously with conversion. The amplification can instead be applied elsewhere than in the converters. In the former case, the energy loss can be a problem because it makes the choice of components critical from this point of view.
The architecture shown in FIG. 1 also has a disadvantage if less than the whole of an exchange is to be equipped with the circuit shown. In particular, the above architecture is not suitable if the exchange is under-equipped, for example if it has only eight input channels and eight output channels. This is because, even for a small number of inputs, although only a single input block would be required, all the star couplers would be needed. The star couplers are not very costly in themselves, but organizing the connections leads in practice to providing fifteen to twenty racks to house all of the equipment. All the racks are still needed if the exchange is under-equipped, for reasons of standardization, and even if each of them is three-quarters empty. There is therefore a problem of mechanical arrangement and overall size.
An alternative solution would be to use only one star coupler, which would receive all the signals to be processed. In this case it would nevertheless be necessary to equip all the input blocks, in each case with a smaller number of positions. For example, the first star coupler could be chosen and all the positions of rank 1 of M=16 input blocks equipped accordingly. This would also make it necessary to provide protection in the form of a redundant converter in these M input blocks. This redundancy would be M times greater than that of the first under-equipped version because each input block would have to include a redundant converter. A solution of this kind therefore requires excessive hardware when it is under-equipped, in addition to the losses of 29 dB.
It would still be possible to design specific architectures, but these would have the drawback of requiring complex design calculations for each under-equipped situation and virtually insoluble logistical problems for any after sales service organization confronted with such a disparate installed base.
In one embodiment, shown in FIG. 2, the architecture of the input blocks is modified to provide a systematic solution to the under-equipment problem. In the input blocks, instead of all the converters converting the input signals to a single wavelength xcex1, they convert them to P different wavelengths. In this case a P by P star coupler, or in one example a 2P by 2P (16 by 16) star coupler, enables a modular structure to be adopted which suits customer needs much better.
The above architecture nevertheless has a major drawback, namely the presence of at least one redundant converter IWTp1. This is because this redundant converter must be able to take over on failure of any of the converters IWTxcex1 to IWTxcex8. Two technologies are feasible. Either the redundant converter is in fact made up of P (8) switchable converters or it is a converter whose frequency can be tuned. On the one hand, the cost of this redundant converter is much greater than the cost of the redundant converters of the FIG. 1 solution. On the other hand, the reliability of the input block would be very much lower precisely because the sources of the tunable converters, which in practice are lasers, are the least reliable components of the converters. In the final analysis, the reliability of the input block would be divided by P, i.e. here divided by eight.
This is because one of the redundant converters could itself break down. If one of the converters should break down, all of the redundancy breaks down. The redundant converter that breaks down might not be the same as a converter of the input block to be replaced by the redundant converter. In practice, the input block would be down P times more frequently. Also, the energy loss is of the same order of magnitude as in FIG. 1: it is still 29 dB.
The object of the invention is to remedy the above disadvantages and to propose an architecture which caters for modular design, and in particular for under-equipment, but makes it less critical by reducing line losses at the time of distribution. The invention also aims to reduce the cost of the input blocks without compromising their reliability.
The idea of the invention is to adopt a solution of the FIG. 2 type except that the redundant converter is not substituted exactly for a converter that is down. To the contrary, in accordance with the invention the redundant converter has to color the optical signal with a redundancy wavelength different from the P wavelengths. The redundancy wavelength is different from all the wavelengths normally distributed by an input block.
Also, the output switch of the input block and the input switch of the output block are eliminated. The star couplers cross connect P+1 input channels to P output channels instead of cross connecting P input channels to P output channels. The star couplers can optionally implement a multiple n of this type of cross-connection. It will be shown that with this approach the additional energy loss in the star coupler is much less than the reduction in losses which results from the elimination of the output switches of the input block and the input switches of the output block.
The hardware saving is therefore doubled, on the one hand because the unnecessary switches are no longer present and on the other hand because the amplification is less critical. It will be shown that the remainder of the switching system remains much the same. The core C must be modified to perform P+1 by P switching instead of P by P switching. This modification is minimal, however. In practice, the switching control unit must allow for information emanating from the input blocks indicating whether a redundant converter is operating or not in those blocks, in addition to the orientation commands OR. This information is already available in the prior art, however, in particular to advise the after sales service department of the need to intervene sooner or later on equipment whose redundant circuits have been switched in because a standard circuit is down. The invention simply uses this facility of the control unit to perform the distribution.
The invention therefore provides an optical line distribution frame with a redundant optical architecture, said distribution frame including:
N input ports for receiving signals on N optical lines,
M primary wavelength converter blocks each connected on the input side to P=N/M input ports and each producing on the output side optical signals at P different wavelengths,
a frequency domain cross-connection and space switching core connected on its input side to the outputs of the primary converter blocks and including modules for switching connections between input channels and output channels of said module,
M secondary wavelength converter blocks each connected on its input side to output channels of said core and on its output side to P=N/M output ports, and wherein the primary and/or secondary converters include converter circuits which are redundant compared to the number of signals with different wavelengths to be protected against failure of one of them and:
a primary converter block includes a redundant converter circuit for converting one of the P signals received at the input to a signal at a wavelength xcexP+1 different from the P wavelengths and P+1 outputs, and/or
a secondary converter block includes a redundant converter circuit for converting one of the P+1 signals received at the input with any of the wavelengths managed by the primary converter blocks and P outputs.
The invention will be understood better after reading the following description and examining the accompanying diagrammatic drawings. The drawings are provided exclusively by way of non-limiting example of the invention.